This invention relates to a dry refractory (i.e., a monolithic refractory installed in dry powder form without the addition of water or liquid chemical binders), particularly a lightweight dry refractory with superior thermal insulation qualities.
Refractories are used as working linings of metal processing and transfer vessels to contain molten metal and slag and the associated heat and gases. These linings typically are consumable materials that are eroded or otherwise damaged by exposure to the conditions within the vessel. When a certain amount of consumption of or damage to the lining has occurred, metal processing must be haltedxe2x80x94sometimes for an extended timexe2x80x94in order to repair or replace the refractory lining. The frequency of these interruptions is determined by the rate at which the process consumes the lining. The duration of these interruptions is dependent on the consumption rate and whether it is possible to repair localized damage to the lining without removing the undamaged portions and replacing the entire lining.
Refractories also are used as secondary or backup linings of vessel working linings. Although these secondary linings are not exposed to molten metals or slags under ideal operating conditions, they must be capable of containing molten metals and slags that penetrate the working lining as a result of erosion, crack formation, or other damage.
Refractories also are used to insulate vessels and other structures used in metal processing and other operations carried out at elevated temperatures. These refractories generally are selected for their heat containment capabilities rather than their resistance to molten metals and slags.
Factors important in refractory selection include the operating conditions of the application, speed and ease of installation and repair, insulating value, and cost. The operating conditions include the predicted chemical and thermal environment to which the refractory will be exposed. For molten metal containment applications, the chemical and thermal environment may be affected by (1) the boundary conditions relating to the dimensions of the shell and the desired capacity of the molten metal pool, (2) the identity and physical properties of the metal, and (3) the expected operating environment of the vessel, including its rated capacity and the presence of features such as oxygen injection, plasma torches, and water or air cooling devices.
Refractories typically are available in the form of bricks, blocks, refractory plastics, ramming masses, refractory castables and dry refractories. Installation and repair of brick and block linings are likely to be costly and slow. Bricks and blocks also must be assembled to avoid gaps at the joints, a time-consuming task requiring skilled craftsmen. Even when the bricks and blocks are carefully fitted together, gaps remain which may allow molten metals and slags to penetrate the lining. Refractory bricks and blocks may have a short life (high consumption rate) and may require removal and replacement of the entire lining when only a portion of the lining is eroded or damaged. This increases the cost of repair and greatly increases downtime.
Conventional refractory plastics and ramming mixes also may have a high consumption rate and may require removal and replacement of the entire lining when only a portion of the lining is eroded or damaged. Castable refractories potentially have a longer life (lower consumption rate) and lower operating and maintenance costs when compared to the prior lining materials. These materials offer the potential for longer life and easier, faster, less expensive installation and maintenance when compared to lining materials typically applied. For example, damaged portions of a castable lining generally can be repaired without removal and replacement of the entire lining.
Installation of a castable refractory lining requires onsite mixing with the attendant mixing equipment, water source, skilled labor and supervision costs, and risk of mixing errors. The quality of the castable lining depends, among other things, on the casting water added, the mixing and vibration techniques used, and the skill of the installers. Transporting the mixed wet castables to the job site may be time consuming, awkward and inconvenient. Installation may require forming, which increases installation time and cost. Dryout of a castable lining at elevated temperatures is needed to remove the added moisture before the lining can be cured and placed into service. Heating of the castable refractory during dryout also increases energy costs.
Conventional refractories and castable refractories are prone to crack formation. Some cracks that form can extend completely through the lining from the hot face (molten metal side) to the cold face (steel shell side). When cracks of this nature occur, the possibility of molten metal and/or slag penetrating via these cracks to an outer shell of the vessel exists. When this occurs, the molten materials can burn through the shell, which may result in extensive damage to equipment and/or injury to personnel. A burn-through of this type can cause long, unscheduled delays in the operation to make repairs to the lining, steel shell and structure, and any surrounding equipment.
Dry refractories are unbonded monolithic materials that are capable of forming strong ceramic bonds at a controlled rate in predetermined temperature ranges and do not contain water or liquid chemical binders. They typically are installed by vibrating, compacting or otherwise de-airing the free-flowing material without addition of water or liquid chemical binders. Dry refractories are easy to install and repair because no mixing is required. Installation of dry refractories is faster and less expensive than installation of castable refractories. In addition to the absence of a mixing step, vessel downtime is reduced because dryout of the lining is not required before a new or repaired lining is placed into service.
The chemical and mineralogical composition of dry refractories can be chosen to be resistant to the specific types and temperatures of metals and slags inherent to a metal containment process. In particular, the refractory can be designed to form strong ceramic bonds in predetermined temperature ranges and at controlled rates of formation. Progressive bond formation, which is influenced by time, temperature, and atmosphere, occurs in response to operating conditions in the immediate environment of the composition. Regions exposed to temperatures above the activation temperature of the bonding agent form strong ceramic bonds while regions exposed to lower temperatures form fewer and weaker bonds.
When ceramic bonding of properly selected dry refractories occurs in this manner, the bonded portion of the material becomes dense and hard and is chemically and physically resistant to penetration of both molten metal and slag. Any portion of the dry refractory that remains below the critical temperature for the formation of ceramic bonds remains as an unbonded monolithic material that does not exhibit brittle behavior or cracking tendencies. The presence of a region of unbonded refractory under normal operating conditions provides improved absorption of mechanical stresses, which may extend the operating life of the vessel lining. Progressively bonding dry refractories have excellent resistance to crack propagation because they can form a barrier to any molten metal and slag that penetrates through the bonded region of the refractory and into the unbonded or fluid region.
For example, a dry refractory may be selected so that regions adjacent to the heat source (e.g., a hot face of a process vessel or an intrusion of molten metal and slag into the refractory lining) form strong ceramic bonds and regions furthest from the heat source remain in an unbonded fluid state until the temperature exceeds the critical temperature, with partial bonding in the intermediate regions. The rigid bonded refractory is chemically and physically resistant to penetration of both molten metal and slag. The unbonded dry refractory exhibits fluid properties that enable it to absorb and distribute local stresses without formation of cracks but is capable of forming strong ceramic bonds upon exposure to more severe operating conditions, e.g., the penetration of molten metal or slag through the bonded portion of the lining.
Refractories with greater insulating capabilities are in demand to conserve energy and reduce energy-related costs in metal processing and similar operations. Although all refractories have some insulating value, so-called insulating refractories (refractories with greater insulating value than typical refractories), such as insulating castable refractories, insulating refractory gunning mixes, and insulating moldable refractories, generally include moisture or liquid chemical binders in their as-installed state before dryout. They also typically have an open porosity that provides low resistance to molten metals and slags. Conventional dry vibratable refractories have excellent resistance to molten metals and are able to absorb mechanical stresses, but large quantities (weights) of material are necessary to form the thick walls typically required to achieve a desired insulating value.
In view of the disadvantages of the prior art, a need exists for a dry refractory that is easy to install, reduces downtime, and provides superior insulating value.
It is an object of the invention to provide a dry refractory with insulating value at least as good as insulating brick and insulating castable refractories with faster installation in similar applications. It is a further object of the invention to provide such a dry refractory with greater resistance to molten metal and slag than conventional insulating refractories.
It is another object of the invention to provide an insulating refractory that does not contain and can be installed without the addition of moisture or liquid chemical binders such that the downtime associated with installation can be reduced.
It is yet another object of the invention to provide an insulating refractory that responds to changing thermal conditions after installation.
It is still another object of the invention to provide a dry refractory that combines the molten metal resistance and stress relief of conventional dry vibratable refractories with superior insulating value. It is a further object of the invention to provide such superior insulating value in a dry refractory of lighter weight than conventional dry vibratable refractories.
The foregoing objects are achieved in a dry refractory composition including filler lightweight material and matrix material. The composition also may include a dense refractory aggregate, a dust suppressant, a bonding agent, or a combination of these.
The filler lightweight material is an insulating refractory material. Preferably, the insulating refractory material is selected from perlite, vermiculite, expanded shale, expanded fireclay, expanded alumina silica hollow spheres, bubble alumina, sintered porous alumina, alumina spinel insulating aggregate, calcium alumina insulating aggregate, expanded mulllite, cordierite, and anorthite.
The matrix material is a fine granular refractory material. Preferably the fine granular refractory material is selected from calcined alumina, fused alumina, sintered magnesia, fused magnesia, silica fume, fused silica, silicon carbide, boron carbide, titanium diboride, zirconium boride, boron nitride, aluminum nitride, silicon nitride, Sialon, titanium oxide, barium sulfate, zircon, a sillimanite group mineral, pyrophyllite, fireclay, carbon, and calcium fluoride.
The composition also may include a dense refractory aggregate, which may be selected from calcined fireclay, calcined Chamotte, a sillimanite group mineral, calcined bauxite, pyrophyllite, silica, zircon, baddeleyite, cordierite, silicon carbide, sintered alumina, fused alumina, fused silica, sintered mullite, fused mullite, fused zirconia, sintered zirconia mullite, fused zirconia mullite, sintered magnesia, fused magnesia, sintered spinel, and fused spinel refractory grog. During installation, the composition contains only incidental moisture, i.e., less than about 1 percent by weight water. The filler lightweight material may be present in an amount from about 15 to 85 volume percent, the matrix material in an amount from about 5 to 50 volume percent, and the dense refractory aggregate in an amount from about 0.1 to 40 volume percent. The composition also may include a dust suppressant in an amount from about 0.1 to 3 volume percent or a bonding agent in an amount from about 0.1 to 15 volume percent.
The composition may form bonds in response to changing thermal conditions. This bond formation may be accomplished by high temperature ceramic bonding of the filler lightweight material, matrix material, and dense refractory material upon exposure to a predetermined temperature range.
Preferably, the composition progressively forms strong ceramic bonds upon exposure to temperatures in predetermined ranges. For example, a first portion of the composition may form strong ceramic bonds upon exposure to temperatures in a first predetermined range and a second portion of the composition may remain in an unbonded fluid form upon exposure to temperatures in a second predetermined range.
The bonding agent, which may be heat activated, preferably is nonliquid at room temperature. The bonding agent may be an organic bonding agent selected from phenolic resin, furan resin, and pitch or an inorganic bonding agent selected from boron oxide, boric acid, cryolite, a fluoride salt, a silicate compound, a phosphate compound, calcium silicate cement, calcium aluminate cement, boron carbide, Sialon, fluorspar, magnesium chloride, fireclay, ball clay, kaolin, and refractory frit. The dust suppressant may be selected from lightweight oil, kerosene, and organic polymer.
In a preferred embodiment, the refractory composition may include filler lightweight material in an amount from about 15 to 85 volume percent, matrix material in an amount from about 5 to 50 volume percent, dense refractory aggregate in an amount from about 0.1 to 80 volume percent, a heat activated bonding agent an amount from about 0.1 to 15 volume percent, and a dust suppressant in an amount from about 0.1 to 3 volume percent. More preferably, the filler lightweight material may be present in an amount from about 50 to 80 volume percent, the matrix material in an amount from about 10 to 30 volume percent, the dense refractory aggregate in an amount from about 0.1 to 40 volume percent, the heat activated bonding agent in an amount from about 0.1 to 10 volume percent, and the dust suppressant in an amount from about 0.25 to 1.6 volume percent.
In another preferred embodiment, the refractory composition may include filler lightweight material in an amount sufficient to achieve a predetermined insulating value and matrix material in an amount sufficient to achieve good resistance to a predetermined chemical and thermal environment. The composition also may include dense refractory aggregate in an amount sufficient to maintain the structural integrity of the composition in the predetermined chemical and thermal environment, a dust suppressant in an amount sufficient to control visible and respirable dust during installation of the composition in dry powder form, or a heat activated bonding agent in an amount sufficient to form strong bonds within the composition.
The present invention also includes a refractory composition including the following ingredients in approximate percent by volume:
More preferably, the composition includes the following ingredients in approximate percent by volume:
The present invention includes a refractory composition including filler lightweight material in an amount from about 15 to 85 volume percent selected from perlite, vermiculite, expanded shale, expanded fireclay, expanded alumina silica hollow spheres, bubble alumina, sintered porous alumina, alumina spinel insulating aggregate, calcium alumina insulating aggregate, expanded mullite, cordierite, anorthite, and insulating refractory material; and matrix material in an amount from about 5 to 50 volume percent selected from calcined alumina, fused alumina, sintered magnesia, fused magnesia, silica fume, fused silica, silicon carbide, boron carbide, titanium diboride, zirconium boride, boron nitride, aluminum nitride, silicon nitride, Sialon, titanium oxide, barium sulfate, zircon, a sillimanite group mineral, pyrophyllite, fireclay, carbon, calcium fluoride, and a fine granular refractory material capable of imparting chemical and thermal resistance to the composition. The composition further may include dense refractory aggregate in an amount from about 0.1 to 80 volume percent selected from calcined fireclay, calcined Chamotte, a sillimanite group mineral, calcined bauxite, pyrophyllite, silica, zircon, baddeleyite, cordierite, silicon carbide, sintered alumina, fused alumina, fused silica, sintered mullite, fused mullite, fused zirconia, sintered zirconia mullite, fused zirconia mullite, sintered magnesia, fused magnesia, sintered spinel, and fused spinel refractory grog; a dust suppressant in an amount from about 0.2 to 3 volume percent selected from lightweight oil, kerosene, and organic polymer; or a heat activated bonding agent in an amount from about 0.1 to 15 volume percent selected from boron oxide, boric acid, cryolite, a fluoride salt, a silicate compound, a phosphate compound, calcium silicate cement, calcium aluminate cement, boron carbide, Sialon, fluorspar, magnesium chloride, fireclay, ball clay, kaolin, refractory frit, phenolic resin, furan resin, and pitch.
In another embodiment of the invention, an installed refractory composition may include filler lightweight material and matrix material, with the composition being substantially free from water and liquid chemical binders during and immediately after installation. At least a portion of the installed refractory may retain fluid properties to relieve mechanical stresses.
The present invention also provides a method of making a refractory composition, including the steps of:
selecting filler lightweight material from perlite, vermiculite, expanded shale, expanded fireclay, expanded alumina silica hollow spheres, bubble alumina, sintered porous alumina, alumina spinel insulating aggregate, calcium alumina insulating aggregate, expanded mullite, cordierite, anorthite, and insulating refractory aggregate;
selecting matrix material from calcined alumina, fused alumina, sintered magnesia, fused magnesia, silica fume, fused silica, silicon carbide, boron carbide, titanium diboride, zirconium boride, boron nitride, aluminum nitride, silicon nitride, Sialon, titanium oxide, barium sulfate, zircon, a sillimanite group mineral, pyrophyllite, fireclay, carbon, calcium fluoride, and fine granular refractory aggregate capable of imparting chemical and thermal resistance to the composition; and
blending the selected filler lightweight material in an amount sufficient to achieve a desired insulating value with an amount of matrix material sufficient to achieve good resistance to a predetermined chemical and thermal environment. The blending step preferably is carried out in the absence of added water or liquid chemical binders.
The method also may include the steps of selecting a dense refractory aggregate from calcined fireclay, calcined Chamotte, a sillimanite group mineral, calcined bauxite, pyrophyllite, silica, zircon, baddeleyite, cordierite, silicon carbide, sintered alumina, fused alumina, fused silica, sintered mullite, fused mullite, fused zirconia, sintered zirconia mullite, fused zirconia mullite, sintered magnesia, fused magnesia, sintered spinel, and fused spinel refractory grog, and adding the dense refractory aggregate in an amount sufficient to maintain the structural integrity of the composition in the predetermined chemical and thermal environment. In addition, the method may include the steps of selecting a dust suppressant and adding the dust suppressant to the composition in an amount sufficient to control visible and respirable dust during installation.
The invention further provides a method of installing an insulating refractory lining, including the steps of selecting an insulating refractory composition in powder form, pouring the dry powdered composition into a void, and de-airing the powdered composition. The de-airing step may include the step of compacting the composition. The step of selecting an insulating refractory may include the step of selecting a refractory composition. This composition may include filler lightweight material selected from perlite, vermiculite, expanded shale, expanded fireclay, expanded alumina silica hollow spheres, bubble alumina, sintered porous alumina, alumina spinel insulating aggregate, calcium alumina insulating aggregate, expanded mullite, cordierite, and anorthite; matrix material selected from calcined alumina, fused alumina, sintered magnesia, fused magnesia, silica fume, fused silica, silicon carbide, boron carbide, titanium diboride, zirconium boride, boron nitride, aluminum nitride, silicon nitride, Sialon, titanium oxide, barium sulfate, zircon, a sillimanite group mineral, pyrophyllite, fireclay, carbon, and calcium fluoride; and a dust suppressant. The composition also may be selected to include dense refractory aggregate selected from calcined fireclay, calcined Chamotte, a sillimanite group mineral, calcined bauxite, pyrophyllite, silica, zircon, baddeleyite, cordierite, silicon carbide, sintered alumina, fused alumina, fused silica, sintered mullite, fused mullite, fused zirconia, sintered zirconia mullite, fused zirconia mullite, sintered magnesia, fused magnesia, sintered spinel, and fused spinel refractory grog, alone or in combination with a heat activated bonding agent.
One embodiment of the present invention is useful primarily in insulating or heat containment applications, although it also may be suitable for containment of less corrosive metals such as copper or aluminum. Denser (but still lightweight compared to conventional refractories), more corrosion resistant refractories of the present invention are suitable for containment of more corrosive molten metals such as iron and steel.
These and further objects of the invention will become apparent from the following detailed description.